1. Field of the Invention
This invention relates to an improved process for removing free liquid from a chilled hydrocarbon-rich gas stream flowing to a dryer. More particularly, this invention relates to an improved process for removing free liquid, principally water and heavier liquid hydrocarbons having boiling points substantially in excess of that of ethylene, from chilled hydrocarbon-rich gas streams that flow to an ethylene plant primary dryer.
2. DESCRIPTION OF THE PRIOR ART
Drying gaseous hydrocarbon-rich streams is a common operation in hydrocarbon processing plants such as ethylene plants, where a gaseous hydrocarbon mixture is separated into several product and by-product streams at temperatures often as low as -150.degree. F.; in natural gas plants, where heavy components and sometimes inert components such as nitrogen are separated at temperatures sometimes lower than -200.degree. F.; in gas treatment plants, where natural gas must be dried to meet pipeline moisture specifications; and in ammonia plants, where a gaseous hydrocarbon mixture is dried before undergoing low temperature processing.
Drying gaseous hydrocarbon-rich streams is commonly carried out by absorption processes, e.g., gas absorption processes using a hygroscopic solvent such as ethylene glycol, or by adsorption processes in which the water vapor being removed from the gas is adsorbed on the surface of a solid adsorbent or desiccant, such as molecular sieve, alumina, silica gel or the like, which have a high affinity for water and liquids.
In typical absorption systems, dry solvent flowing downwards in an absorber contacts upflowing process gas which contains water vapor. The dry solvent absorbs water from the gas, forming a dry gas stream and a water-rich solvent stream. The water-rich solvent is then passed to a regenerator and heated to remove absorbed water from the solvent. Regenerated solvent, after cooling, is recycled to the absorber.
In a typical adsorption system, water vapor containing process gas flows through a fixed bed of desiccant particles, which keep adsorbing water until the bed surface becomes saturated. When this occurs, the process gas flow is switched to another desiccant bed and the water-saturated dessicant bed is regenerated by flowing dry, heated gas through it to remove adsorbed water.
How well both absorption and adsorption dryers perform depends primarily on the quantity of water contained in the process gas being dried. In the typical adsorption system, an increase in the water content of the process gas stream increases solvent circulation rate, solvent cost and regeneration cost, and can necessitate an increase in equipment size. Similarly, in the typical adsorption system, an increase in the water content of the process gas stream reduces the length of the adsorption cycle and, consequently, increases the frequency of regeneration. This in turn reduces the useful life of the desiccant, and increases the costs of regeneration.
Hydrocarbon process gases can contain water in two forms--as water vapor and as "free water". Free water is water in the liquid state, usually in the form of liquid droplets. Neither form is desirable at the dryer inlet, and both are usually minimized to the greatest practicable extent prior to the process gases' entering a dryer. One way of minimizing water vapor in a hydrocarbon process gas is to cool the gas to as low a temperature as possible short of that at which hydrates are formed. Hydrates are loose solid chemical compounds of hydrocarbons and water which, when formed in processing equipment such as pipelines, heat exchangers and fractionation columns, behave like ice, causing equipment blockages and reducing effective equipment capacity. In most cases, this low temperature will typically be between about 45.degree. F. and about 75.degree. F. As the temperature is lowered, water vapor is condensed from the gas and converted to free water. The process gas containing condensed free water then flows to a vapor-liquid separator, usually a knockout drum equipped with a mist eliminator, in which free water and other condensed liquids in the form of continuous liquid or large drops of liquid (i.e., droplets which are capable of being knocked out by standard mesh pads) are separated from the process gas.
In many instances, the quantity of free water present as fine drops is significant, and there is an incentive to remove these fine drops before drying the gas. Two techniques have been used for this purpose: either a fine mesh pad is installed in the knockout drum, or a filter-coalescer is installed in the process gas line which leads from the knockout drum to the dryer. However, fine mesh pads are sensitive to plugging by any solids or heavy viscous liquids carried over by the process gas. Further, the pressure drop across the mesh pad will increase, sometimes substantially, as plugging progresses. Filter-coalescers, although they can remove finer liquid drops than can fine mesh pads, are also sensitive to plugging and pressure build-up. For this reason, filter-coalescer units are frequently installed in parallel, so that one can be cleaned while the other remains on-line.
Solving the plugging in this fashion, however, introduces others: considerable additional capital expenditures are required, and a significant pressure drop is added to the system. This pressure drop, in turn, increases compressor power requirements and, ultimately, operating costs.
In hydrocarbon processing plants in which neither a fine mesh pad nor a filter-coalescer is installed, there is no significant problem as long as little free water exists in the form of fine droplets in the hydrocarbon-rich gas stream flowing to the dryer. Serious problems are created, however, if appreciable amounts of free water reach the dryer. Absorption cycles and desiccant life can be considerably reduced, thus causing significant increases in operating costs for the purchase of new desiccant and for more frequent regenerations.
Accordingly, there are always incentives to minimize the quantity of water contained in hydrocarbon-rich process gases flowing to a dryer. A low cost method for removing fine liquid droplets from a hydrocarbon-rich process gas stream before the stream reaches the dryer could substantially reduce capital costs in any new plant being built in which such streams are dried, and would also eliminate the problems, with their attendant costs, which arise in new or existing plants whenever free water arrives at the dryer.
Hence, it is an object of this invention to provide an improved process for removing free liquid from a hydrocarbon-rich gas stream flowing to a dryer.
It is also an object of this invention to provide an improved process for removing free liquid, principally water and heavier liquid hydrocarbons having boiling points substantially in excess of that of ethylene, from hydrocarbon-rich gas streams that flow to an ethylene plant primary dryer.
A further object of this invention is to provide an improved process for removing free liquid from a hydrocarbon-rich gas stream flowing through a chiller to a vapor-liquid separator and ultimately to a dryer in which the need to install fine mesh pads or filter-coalescers to remove water before the gas stream reaches the dryer is eliminated.
These and other objects, as well as the nature, scope and utilization of the invention, will become readily apparent to those skilled in the art from the following description, the drawing, and the appended claims.